The present invention relates to a strong-convergent type charged particle acceleration/deceleration tube suitable to compose a high-current ion beam generator.
Conventionally, an acceleration/deceleration tube as shown in FIGS. 3(a) and 3(b) is used which comprises a set of electrodes of hollow cylinder or disk type. This tube is widely used for ion beam generator. FIG. 3(a) is a schematic construction of the ion beam generator and FIG. 3(b) is an explanation showing the shape of an electrode 33 (34, 35) viewed in the direction of z-axis as a common axis. In FIG. 3(a) an ion source 31, an extraction electrode 32, and an acceleration tube 30 are aligned coaxially along the z-axis. The acceleration tube 30 comprises three electrodes 33, 34 and 35 of hollow cylinder type. An electric power is applied to work the ion source 31 and the extraction electrode 32 from a high voltage power source (not shown). A DC voltage is applied to the electrodes 33, 34, and 35 from a high voltage power source 36 after being divided by three resistors serially connected. The voltages respectively applied to the electrodes 33, 34 and 35 are set gradually lowered, so that the electrodes 33, 34 and 35 serve to accelerate the ions. The ions generated by the ion source 31 are taken out in a beam-like form with the potential difference between the ion source 31 and the extraction electrode 32, so that they are accelerated along the z-axis by passing through the inner parts of the accelerating electrodes 33, 34 and 35.
On the other hand, in order to reduce the speed of the ions, the voltage of the high voltage power source in the ion beam generator in FIG. 3(a) is reversed. Then, the electrodes 33, 34 and 35 serve to reduce the speed of the ions. The ions generated by the ion source 31 are once taken out in a beam-like form but are decelerated along the z-axis by passing through the inner parts of the reduction electrodes 33, 34 and 35.
In the above-mentioned acceleration/deceleration tube, it is disadvantageous that the ion beams of high current cannot be efficiently obtained, because the electrodes 33, 34 and 35 of the hollow cylinder have weak convergent action and cannot converge the beam strongly. Further, in case where the ion beams of high current are running, it is believed that the ion beams are diverged along the x-axis or y-axis due to the Coulomb's force between the ions in the beam.